A circuit structure of conventional ground fault detector for subscriber circuits utilizing a DC/DC converter into the battery feed circuit is shown in FIG. 1. In FIG. 1, the reference numeral 1 designates a DC/DC converter; 2, a comparator; 3, transformer for power feeding; 4, a subscriber telephone set.
A DC/DC converter 1 is a DC voltage converting circuit for boosting -24V voltage (system power supply of exchange) up to -48V and a comparator 2 is a circuit for detecting ground fault through comparison of voltage with the reference voltage and sends a supervisory signal to the controller of exchange. If a ground fault is generated on the subscriber lines, a battery feed current I is doubled in case the line resistance is zero. Accordingly, in case a voltage of DC/DC converter 1 is fixed to -48V, a battery current increases. Moreover, the ground fault detection voltage V.sub.E becomes the ground potential, the comparator 2 is inverted, thereby the signal "1" is sent to the exchange controller and an alarm is issued. Even if a ground fault occurs, the battery feed of -48V still supplies -48V continuously, the DC/DC converter 1 requires the battery feed resistors R.sub.T and R.sub.R which are resistive to four times of power dissipation and is also required to have a structure which is resistive to the doubled current. As a result, the DC/DC converter 1 results in not only rise of cost but also increase in size of circuit elements and restriction on mounting of elements.